The term "free flow" in bars and restaurants refers to an unlimited supply of drinks (usually alcohol) for a set price during a certain time. Simply put, you pay once and drink as much as you want during the allotted time.
Every Wednesday, Mrs. B's bar offers a special offer: for just 325 baht, you get an hour of unlimited drinks—the menu includes Aperol Spritz, signature Spritz, Prosecco, and Gin & Tonic.